This invention relates to the generation of fine bubbles and their application in two processes, one being airlift loop bioreactors and the other being particle, in particular, algal floc, separators.
Bubbles of gas in liquid are frequently required in many different applications and usually, but not exclusively, for the purpose of dissolving the gas in the liquid. Like any industrial process, it is generally desired that this be done in the most efficient manner possible. It is widely recognised that one way to achieve efficiency is to reduce the size of the bubbles. The surface area to volume ratio of a smaller bubble is higher. Moreover, the surface tension of a small bubble means that the gas pressure inside the bubble is relatively much higher than in a large bubble. Also small bubbles rise more slowly than large bubbles and they coalesce less quickly so that larger bubbles, that rise to the surface faster, are less quickly formed.
Applications that do not involve gas dissolution exist in oil wells where bubbles rising can transport oil to the surface in certain types of well. Here small bubbles are also advantageous because it takes them longer to coalesce and form the big slugs of gas that are not effective in raising oil. The reason large slugs are not effective is because they have a small surface area compared with small bubbles and it is the surface of bubbles to which articles adhere. It has also been discovered that in the case of particles being lifted by bubbles there is a correlation between bubble size and particle size, in the sense that approximately equal sized bubbles and particles results in good collision efficiency and thus floc formation.
The corollary problem connected with fine bubbles, however, is that they are harder to produce. Reducing the size of the aperture through which the bubble is injected into the liquid is a first step, since it is difficult to form small bubbles through a large aperture. But, even so, a bubble may reach a large size by growing while attached even to a small gas-supplying aperture. Bubble separation is a dynamic process. In any event, such reduction in aperture size is not without cost, because the friction resisting flow of the gas through such a fine aperture, and through the passage leading to the aperture, means that a greater pressure drop is required. The bubble forms once the size of the bubble goes beyond hemispherical and necking-off of the bubble can occur. However, more energy needs to be applied at this stage to finally detach the bubble and generally this is simply achieved by pressing more gas into it increasing its size.
Indeed, generally, bubbles can be no smaller in diameter than the diameter of the aperture through which they are injected, and reducing the size of the bubble increases the energy needed to produce them so that a limit is reached beyond which the efficiency of the system is not improved any further. In fact, getting bubbles anywhere near the size of the aperture (i.e., less than double) would be a significant advancement.
A further problem is that, as bubbles grow beyond hemispherical, the pressure inside them drops. Consequently, two or more bubbles grown in parallel from a common source tend to be unstable beyond hemispherical. What occurs is that, beyond the hemispherical stage, one bubble grows rather more rapidly than an adjacent one (for any of a number of reasons, eg perhaps one is closer to the pressure source and so there is correspondingly less drag and greater pressure to drive the bubble formation). Once there is a size differential there is also a pressure differential with the greater pressure being in the smaller bubble. Consequently, since the bubbles are connected, the smaller bubble inflates the larger one at the expense of its own growth. The result is that, where multiple conduits are connected to a common pressure source, only a few of them end up producing overly large bubbles.
This instability of bubble formation may lead to one of the bubbles growing out of proportion to the aperture size. The necking-off and separation is a dynamic phenomenon and if the unstable bubble grows fast, it may reach a big size before it separates.
Another problem with uncontrolled bubble formation is that colliding bubbles frequently coalesce, so that the extra effort of forming small bubbles is immediately wasted. Ideally, monodisperse bubbles should be provided with sufficient gap between them to prevent coalescing. Indeed, the conditions that lead to coalescing may be dependent on a number of factors connected with a particular site and application, and that, desirably tuning of a bubble generation system should be possible so that the most efficient bubble generation can be arranged.
WO99/31019 and WO99/30812 both solve the problem of fine bubble generation using relatively large apertures by injecting the gas into a stream of the liquid being driven through a small aperture directly in front of the gas exit aperture. The stream of liquid draws the gas into a fine stream, much narrower than the exit aperture for the gas, and fine bubbles ultimately form beyond the small aperture. However, the physical arrangement is quite complex, although bubbles of 0.1 to 100 microns are said to be produced. Furthermore, although the gas exit aperture is large, the liquid into which the gas is injected is necessarily under pressure to drive it through the small aperture which therefore implies that the gas pressure is necessarily also higher, which must mitigate some of the advantage.
Numerous publications recognise that vibration can assist detachment of a bubble or, in the case of EP1092541, a liquid drop. That patent suggests oscillating one side of an annular discharge orifice. The production of liquid drops in a gas matrix can sometimes be regarded as a similar problem to the production of gas bubbles in a liquid matrix.
SU1616561 is concerned with aeration of a fish tank which comprises forcing air through a pipe where apertures open between flaps that vibrate under the influence of the gas motion and produce fine bubbles.
GB1281630 employs a similar arrangement, but also relies on the resonance of a cavity associated with a steel flap to increase frequency of oscillation of the flap and thereby further reduce the size of the bubbles.
U.S. Pat. No. 4,793,714 pressurises the far side of a perforated membrane through which the gas is forced into the liquid, the membrane being vibrated whereby smaller bubbles are produced.
U.S. Pat. No. 5,674,433 employs a different tack by stripping bubbles from hydrophobic hollow fibre membranes using volume flow of water over the fibres.
GB2273700 discloses an arrangement in which sonic vibrations are applied to the air in a sewage aeration device comprising a porous “organ pipe” arrangement, in which the pipe is vibrated sonically by the air flow. The invention relies on vibration of the aerator by virtue of the organ pipe arrangement, losing much of the energy input through inevitable damping by the surrounding water.
DE4405961 also vibrates the air in an aeration device for sewage treatment by mounting a motor driving the air pump on the aeration grid employed, and so that the grid vibrates with the natural vibration of the motor and smaller bubbles result. DE19530625 shows a similar arrangement, other than that the grid is oscillated by a reciprocating arrangement.
JP2003-265939 suggests ultrasonically vibrating the surface of a porous substrate through which a gas is passed into a liquid flowing over the surface.
From the above it is apparent that small bubble generation has application in the sewage treatment industry, in which it is desired to dissolve oxygen in the water being treated. This is to supply respiring bacteria that are digesting the sewage. The more oxygen they have, the more efficient the digestion process. However, a similar requirement exists in bioreactors and fermenters generally where they are sparged for aeration or other purposes. Specifically, the yeast manufacturing industry has this requirement, where growing and reproducing yeast bacteria need constant oxygen replenishment for respiration purposes. Another application is in the carbonisation of beverages, where it is desired to dissolve carbon dioxide into the beverage. A process not looking to dissolve the gas but nevertheless benefiting from small bubbles is in the extraction of hard-to-lift oil reserves in some fields which either have little oil left, or have the oil locked in sand. Indeed, much of the oil in Canada's oil reserves is in the form of oil sand. Bubbling gas up through such oil-bearing reserves has the effect of lifting the oil as the bubbles rise under gravity and bring the oil with them. The bubbles are formed in water and pumped into the well or reserve and the oil is carried at the interface between the gas and water of each bubble as it passes through the reserves. The smaller the bubble, the greater is the relative surface area for transport of the oil.
Another application of bubbles is in particle separation from a liquid suspension of the particles, of which an example application is algal separation. This might be desired for one or both of two reasons. A first reason is to clarify water contaminated with algae. A second reason is to harvest algae grown in water. The process is not limited to algal separation; any mixer comprising solid particles can comprise the use of bubbles which attach to suspended particles and carry them to the surface from which they may be scraped, either to recover the particles or clarify the water. However, attachment of the bubbles to the particles is problematic, particularly if the particles are charged when they may simply bounce off bubbles and not attach to them. Dissolved air flotation (DAF) is a known technique where air is dissolved under substantial pressure into water which, when released into the separation tank immediately releases large quantities of small bubbles. However, there are two issues. The first is the substantial energy requirements to compress and dissolve air in water. The second is the turbulence of the bubbles released into the tank.
It is an object of the present invention to improve upon the prior art arrangements. It is also an object to provide process applications using small bubbles.